The use of a film of chromium nitride (CrN and/or Cr2N) on the external peripheral surface of a sliding member of an internal combustion engine, such as a piston ring, provides high wear resistance. This type of coating is widely used in components for recently developed engines wherein high thermal and mechanical loads are the rule during operation. Such coating is obtained by the physical deposition of metal vapour generated by a cathodic arc source.
However, in engines operating under extremely high load conditions and having a high combustion pressure level, such as those with a turbocompressor, this coating of CrN and/or Cr2N ceramic presents intrinsic fragility leading to the appearance of microfissures in the surface thereof. These microfissures propagate and the connection thereof leads to a loss of small pieces of the coating (a phenomenon called “fragmentation”), generating empty spaces in the surface thereof and, in some drastic cases, scratching of the engine liners. The lack of coating on some portions of the ring exposes the base metal which commences experiencing severe wear and then the performance of such a component diminishes rapidly.
A further large obstacle for the wide application of cathodic arc plasma deposition is the presence of macroparticles in the coating. These macroparticles induce defects in the coating which result in premature damage to the generally hard and corrosion-resistant coatings. The prejudicial effect of the formation of macroparticles includes the local loss of coating adhesion, surface corrugation and surface contraction formation due to a macroparticle autorepulsion mechanism, none of which are relieved by tribological applications.
Another type of coating composed of coatings of doped TiN is used in dry cutting operations, by virtue of its high oxidation temperature and abrasive wear resistance. However, as may be observed for the chromium ceramic coating, the presence of macroparticles due to the cathodic arc process endangers the corrosion and fragmentation resistance of this coating for piston ring applications.
There are various cases of patents belonging to the prior art revealing piston rings or other sliding members having a coating comprising a nitride (metal element+nitrogen (N)) and at least one additional metal element, however they disclose the same composition as the present invention and, as a consequence, have the same performance in relation to fragmentation resistance, wear resistance and degree of abrasion resistance as a function of the advantageous properties of a considerably reduced internal stress level and number of macroparticles per square millimeter of the average cross-sectional area of the coating.
British patent GB 2259715 discloses a piston ring having a surface coating of metal and metal carbide/nitride, together with the process for manufacturing the same.
The process patented uses an ion arc galvanising apparatus wherein a metal not having the capacity to form a carbide or nitride, such as cobalt, nickel or molybdenum, is defined as a first target, a metal having the capacity to form a carbide or a nitride, such as silicon, titanium, vanadium, chromium, iron, zirconium, niobium or tungsten, is defined as a second target, and nitrogen, acetylene or methane is fed as a process gas to form a film on a piston ring body, being an article to be coated, in this manner furnishing a piston ring comprising a hard film formed on at least one sliding external face of the body of the piston ring. Such hard film comprises a mixed structure formed by the metal not having the capacity to form a carbide or a nitride and a carbide or nitride of the metal having the capacity to form a carbide or nitride. On the piston ring of such an invention, the hard film has such good adhesion to the material of the body or film hardness that fissuring or detachment of the film does not tend to occur, even when there is formed a film having a great thickness. The carbide or nitride in the hard film contributes to the realisation of superior wear resistance. By virtue of the fine structure of the hard film, the wear of the opposing material may be reduced, there tends to be no loss of film lubrication and corrosion resistance may be improved. Subsequent to the hard film having been formed nitriding may be realised.
International patent application WO2008/152104 discloses a piston ring comprising a support material and a wear resistant coating composed of a ternary system A-B—N applied using a PVD process, wherein A and B in each case represent an element of the group comprising Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, Si and C, and N represents nitrogen. The thickness of the wear resistant coating is 3 microns.
U.S. Pat. No. 7,160,635 discloses nitrided coatings based on protective Ti—Al—Cr for the protection of substrates operating at moderately elevated temperatures. More particularly, the coatings were developed for the protection of aircraft titanium alloy and components of stationary gas turbines, together with engine components for automotive applications, articles having such coatings, and a method for the production thereof.
U.S. patent application No. 2009/0075114 discloses a method for the manufacture of a protective coating of a hard material on a substrate consisting of a metal or of an electrically conductive ceramic material, for example a tool coated for use in a machine tool or components exposed to high temperature wherein, prior to the deposition of the coating of the hard protective material, the substrate is pretreated by metal ion bombardment of at least one rare earth element, in this manner resulting in the implantation of some of said ions in the said substrate.
U.S. Pat. No. 5,154,433 discloses a piston ring comprising a martensitic stainless steel ring having a nitrided layer formed on a portion of the surface thereof and a titanium nitride coated on the external sliding surface of the ring by an ion galvanisation process. In one of the embodiments of this invention, the titanium nitride film has a nitrogen concentration which is gradually increased in the direction from the upper surface thereof downwards, a film hardness gradually increased in the direction towards the upper surface rendering a portion of the film surface more fragile than the other portion of the film. Furthermore, according to a second embodiment of the invention, the titanium nitride film has a nitrogen concentration which is gradually diminished in the direction towards the upper surface and has a film surface hardness of 1800 HV or less.
U.S. Pat. No. 5,316,321 discloses a non-ferrous piston ring having a hard surface treatment layer. More specifically, the ring is made from a titanium alloy and at least one external peripheral surface and the upper and lower surfaces thereof are formed by a diffused layer of nitrogen and at least one external peripheral sliding surface thereof is coated with a hard film by means of physical vapour deposition on the diffused nitrogen layer.
U.S. Pat. No. 6,372,369 discloses a hard film coating, a sliding member similar to a piston ring covered with a hard film and a method for the manufacture thereof.
In such an invention, a nitrided layer is formed on a piston ring and a hard film is formed by ion arc galvanisation on the nitrided layer on the external circumferential surface. The hard layer has a crystal structure comprising mixed phases of CrN and TiN and contains oxygen in solid solution state in the crystals of CrN and TiN. The hard film comprises chromium from 40 to 75 percent by weight, titanium from 10 to 40 percent by weight, oxygen from 0.5 to 15 per percent by weight, the balance being nitrogen. The crystal particle size lies within 1 μm. The Vickers hardness of the hard film lies in a range from 1300 to 2300. The crystals of CrN and TiN have a preferred surface orientation of (200) or a preferred surface orientation of (111) parallel to a surface which is covered and has a structure in columnar form, being in the form of a column from the base material in the direction of the film surface. Carbon in solid solution state may be contained instead of oxygen, and both the oxygen and carbon may be contained in solid solution state. An underfilm of CrN may be formed between the nitrided layer and the hard film.
U.S. Pat. No. 6,161,837 discloses a piston ring having a hybrid coated face together with a process for manufacturing the ring. A base body of annular steel has a lower layer formed by electrogalvanisation of a peripheral surface of the body with a chromium or titanium nitrided gas. The ring is normally bathed until coating, smoothing and removal of residual metal galvanisation nodules, which yield random microabrasions in the lower layer. An upper layer of metal nitride or chromium ceramic is formed on the lower bathed layer by means of the use of physical vapour deposition means.
U.S. patent application No. US2008/0260478 discloses a PVD coating and in particular a nanoscale multilayer superlattice PVD coating comprising high hardness, low coefficient of friction and increased chemical inertness. The multilayer coating comprises a repeating bilayer represented by (VxMe(i-x))CyN(i-y)/(MezV(1-z))CyN(i-y) wherein 0.1≦x≦0.9, 0.01<y<0.99 and 0.1≦z≦0.9, and Me is a substantially pure metal or a metal alloy.
The composition of the coating through the layers alternates from layer to layer according to a modulated sequence of a V-rich layer and an Me-rich layer. Vanadium is incorporated within the layer composition and was found to act as a lubricating agent during sliding wear. Carbon, also incorporated within the coating, serves to further stabilise the coefficient of friction thereby increasing chemical inertness.
Finally, U.S. patent No. 2012/037493 discloses a coating technology wherein an arc evaporation source is supplied in such manner that the speed of formation of the film is raised, there being induced magnetic lines in the direction of the substrate. The arc evaporation source is provided with: at least one external circumferential magnet disposed in such a manner that the external circumferential magnet surrounds the external circumference of a target and that the direction of magnetisation thereof is in the direction which crosses the surface of the target in an orthogonal manner, and a rear surface magnet disposed on the rear surface side of the target. The rear surface target has a first permanent magnet having a format differing from a ring wherein the polarity thereof is orientated in the same direction as the polarity of the external circumferential magnet, the direction of magnetisation of the rear surface magnet is in the direction crossing the target surface in an orthogonal manner.
There has not been developed thus far a sliding member such as a piston ring having a coating applied by the PVD (physical vapour deposition) process generated by an HiPIMS source wherein such coating is a multilayer composed of a nitride (metal element+nitrogen (N)) and by at least one metal element additional to the nitride which demonstrates excellent performance in relation to fragmentation resistance, wear resistance, and degree of abrasion resistance flowing from the advantageous properties of a low level of internal stress and being free of macroparticles intrinsic to the cathodic arc process.